On the surface everyone would say Germany because they invaded Belgium. But what about other countries? Russia could be blamed for their partial moblisation fueling an European mobilisation, Austria-Hungary could also be blamed for their impossible ultimatum to Serbia knowing full well Serbia would not accept. Perhaps one cannot blame nations but people or ideas: Nationalism, Imperialism, Kaiser Wilhelm II, Conrad von Hötzendorf could all be blamed for the start of WW1. Listed below is but a very brief summary for the popular causes for the war
Nations
-Starting the naval arms race between Britain, antagonising Britain
-Schlieffen plan forced Germany to go to war against France if she wanted to go to war against Russia.
-Infamous blank check to Austria-Hungary
-When Serbia accepted the ultimatum, Wilhelm responded by advocating for an occupation of their capital Belgrade because the Serbians can't be trusted
-Fischer Thesis: Essentially Germany dreamed of becoming a large colonial power thus by 1911 accepted the risk for war and started to prepare for expansion
Austria-Hungary
-They know that the ultimatum was impossible to accept, thus giving Austria-Hungary the excuse to wage war also bringing in Russia
-Conrad, the greatest advocator for a war against Serbia
Russia
-Partial moblisation starting an European mobilisation
-Franco-Russian alliance (1892) forced France to join a Russian war.
-Russian wishes of united slavic people, aligning themselves with Serbia
France
-Gave a 'blank cheque' to Russia, making no attempt to restrain Russia
Britain
-Policy of splendid isolation, Britain could have stopped the war during the July crisis if she was more decisive and authoritarian
Themes
-Russian wishes of united slavic people, aligning themselves with Serbia
Imperialism
-The Russian quest for a hot water port
-Germany's imperialist dreams of a "place in the sun" causing tension in Africa with Britain.
-Germany's ambition for "Mittelafrika"
Alliance System
-The Triple Alliance between Germany, Austria-Hungary and Italy and the Triple Entente between France, Russia and Britain with various other alliances and allegiances.
Militarism
-From Edward Gray 1909 in reference to the Naval race "[Germany navy is a question of] our independence, our very existence would be at stake"
-From Edward Gray's memoirs '[Militarism] Great armaments lead inevitably to war"
In my opinion,
All nations deserve some blame for their responsibility for WWI, never the less Germany deserves the most for starting a European war and Russia deserves great blame in escalating the conflict from a Balkan conflict, to a European conflict.
Whilst Austrian-Hungarian attempted to use the assasiantion of Franz Ferdinard as an excuse to destroy a rebellious neighbor, Germany alone had the power to halt this aggression but it persisted with it's blank cheque further antagonising the Balkan region.
With Kaiser Wilhelm's militaristic and imperialistic ambitions, Germany was convinced that a European war would fulfill its' "place in the sun" or "millteafrika" thus when Serbia refused the ultimatum, Wilhelm persisted in attacking Serbia
Russia, humiliated after the 1856 defeat in the Crimean war and with a populace increasingly rebellious under the Tsar's rule needed foreign policy success. Hence in an attempt to unify the slavic people, lay her support for Serbia. When Austria-Hungary attacked Serbia and encouraged by a blank cheque given to them by France resulted in a Russian moblisation for war; Russia, not Germany moblisised for war thus bringing the Balkan conflict into the European stage with the dragging in of France
Germany panicked over Russian moblisation in 10 days enacted the Schiefflen plan in a desperate attempt to contain a war-ready Russia, thus starting the World War with its attack on neutral Belgium soil.